Pigments are discrete colored particles which, when dispersed in the applicable media, impart color to it. The physical properties of pigments such as the particle size, size distribution, and the crystal structure are some of the parameters that determine the effectiveness and suitability of a particular pigment for a particular application, such as use in an aqueous media, non-aqueous media, or with plastics.
Azo pigments represent an important class of coloring agents used primarily in the manufacture of water-based inks, solvent-based inks, plastics, water-based paints and solvent-based paints. Some commonly utilized azo pigments are diarylide pigments including Pigment Yellow 12, Pigment Yellow 13, Pigment Yellow 14, Pigment Yellow 17 and Pigment Yellow 83. Other examples are disazo pyrazolone pigments, including Pigment Orange 13, Pigment Orange 34, Pigment Red 37, Pigment Red 38, Pigment Red 41 and Pigment Red 42.
Azo pigments are synthesized from various primary aromatic amines by a pigment synthesis process well known in the art, which includes the steps of diazotization and coupling. The diazotization preferably involves treating a primary aromatic amine with nitrous acid and the resulting compound is called diazonium salt. This diazonium salt is reacted with a coupling component to form the azo pigment in a reaction called a coupling reaction. Examples of coupling components are phenols, naphthols, and compounds that contain an enolyable methylene group, such as acetoacetanilides or pyrazalones.
Azo pigments, however, have deficiencies in certain applications and, in general, can be improved upon. For instance, in the graphic art industry, more specifically in multicolor printing, pigments of high color strength (i.e., high tinctorial strength) pure shade, good light fastness, and good flow properties (flow properties are also referred to as rheology) are needed. Moreover, especially in the case of yellow pigments, the ink industry has been able to obtain superior qualities in yellow pigments through the use of diarylide yellow pigments. Diarylide yellow pigments are undesirable, however, because of health hazards associated with dichlorobenzidine, which is used to manufacture the pigment and which is a carcinogen suspect. Therefore, monoazo pigments are used to avoid the health hazards associated with diarylide pigments despite the fact that monoazo dye properties are generally inferior.
One way to improve the properties of pigments is to alter their crystalline structure. It is generally understood by those skilled in the art that pigment particles aggregate or crystallize after the pigment is formed in a pigment synthesis process. It is also known that when a pigment is dispersed in the applicable media in which it is used, to some extent, the physical act of dispersion breaks apart these crystals or particle aggregations (hereinafter referred to simply as "crystals"). If allowed to, however, the pigment will separate from the media and recrystalize into large, aggregate particles. Pigment properties are improved if crystal growth can be inhibited, either prior to the initial formation of the crystals during pigment synthesis, or prior to recrystallization in an applicable media, because smaller crystals impart better pigment properties than larger crystals.
One example of a specific pigment problem due to large crystal size is disclosed in U.S. Pat. No. 4,474,609. There, the inventors explain that certain azo pigments show a certain solubility in some organic solvents or solvent-containing binding agents, which can adversely affect their applicability. For example, when certain pigments are dispersed in varnish binding agents the tinctorial strength increases at first as the dispersing period increases, but eventually levels off. Also, the tinctorial strength decreases upon storing ready-made varnish (i.e., which already includes the pigment dispersed in a media), and tinctorial strength tends to decrease more rapidly at higher temperatures because the pigment particles crystallize to form a large aggregate. The loss in tinctorial strength is in most cases accompanied by a decreasing transparency, and frequently also by an alteration of the shade.
The factor responsible for these undesirable processes in the preparation and storage of those varnishes is believed to be the recrystallization of the dispersed pigment particles in the solvents present. Owing to known physical laws, the small crystal particles having a high surface energy are dissolved, whereas the larger particles already present continue to grow, their ultimate size depending upon the solubility conditions. The resulting large crystal size leads to a loss in tinctorial strength and a reduced transparency.
U.S. Pat. No. 4,474,609 also discloses a recrystallization-resistant monoazo pigment of alleged high tinctorial strength. Col 2, 11. 5-8. The monoazo pigment mixtures are obtained by coupling diazotized amines of the benzedine series onto acetoacetic acid arylamides. Col. 2, 11. 9-11. A percentage of the diazo and/or coupling component contains acid groups, preferably sulfo and/or carboxy groups. Col. 2, 11. 14-16. Subsequently, the reactive acid groups of the product obtained by the coupling are reacted with cation-active quaternary compounds, especially quaternary ammonium or phosphonium compounds. Col. 2, 11. 16-18.
The present invention differs from the prior art in that the crystal growth inhibitor has a molecular structure of such geometry, with a polar group so situated, that it efficiently inhibits the growth of large pigment crystals (i.e., large aggregates of pigment particles). More specifically, the present invention discloses crystal growth inhibitors having a molecular structure similar to that of the azo pigments with which they are used, and containing at least one acid functional group (preferably sulfonic or carboxylic). The crystal growth inhibitors can be made in situ during synthesis of an azo pigment or added during synthesis of the pigment, preferably prior to the inception of pigment crystals.
The crystal growth inhibitor of the present invention does not utilize quaternary compounds, is a monoazo compound and covers a wide field of application, i.e., it can be used with all types of azo pigments and is not limited to diarylide yellow, arylamide yellow, .beta.-naphthol red or arylamide red. It results in significant improvements, such as superior gloss, tinctorial strength and transparency in products such as water-based inks, solvent flexo inks, solvent gravure inks and oleoresenous inks.